The appearance of printed text and/or images on paper can be affected by the presence of a coating on the paper. The coating can contain a mixture of clay, pigment, and binder. When ink is applied to an uncoated paper it is absorbed by the paper. When ink is applied to coated paper it sits on the coating. This attribute allows ink printed on coated paper to retain a crisp edge. As a result, coated paper generally produces sharper, brighter images and has better reflectivity than uncoated paper.
In producing a coated paper, the coating is first applied over a base paper, and then the coated base paper is consolidated in a calendering operation to make it more suitable for printing. Calendering affects the surface, as well as the whole paper structure, of a coated paper in many ways. For example, it reduces the roughness of the paper. Coated paper roughness depends particularly on the deformation of the fiber network during calendering. A decrease in roughness is often accompanied by an increase in gloss. Paper gloss, which is a surface related paper property, depends mainly on the deformation of the coating layer structure in calendering.
Calendering also affects the structure and characteristics of the base paper. For example, calendering can lead to the loss of opacity; a decrease in stiffness, and a reduction in strength of the base paper. This is especially true when the coated paper has been harshly calendered.
The coating may be finished in the calendering process to a high gloss, a gloss, a dull, or a matte (not glossy) finish. Other slight variations on these finish categories are possible. Coated paper is also categorized into grades by its brightness and gloss level. The grades can include premium coated paper (the brightest and highest quality grade of coated papers), coated #1, coated #2, coated #3, coated #4, coated #5, coated board, coated laser paper, coated CIS (coated 1-side), coated reply card, and coated SCA (Super Calendered Type A).
Improvements in producing the different grades of coated paper and the coating compositions used to form the coated paper continue to be a desire in the art.